Oxygen dissolved in the feed water, dissolved copper, and further corrosion products carried along by the feed water have recently been identified as the promoters of corrosion in steam generators of pressurized-water reactor installations. Consequently, there have recently been great efforts to keep the oxygen content in the feed water as low as possible, to eliminate dissolved copper by removing all pipes made of copper alloys from the feed water circulation system, and to separate out corrosion products before they reach the steam generator. These efforts are all the greater because the replacement of steam generators in a power station in the 1,000-MW class results in costs of approximately 100 million dollars. In addition, a reduction in the oxygen content of the feed water lessens the corrosion in the feed water line and the preheaters.
Oxygen is dissolved in the feed water or in the condensate when air comes in contact with the water. For example, this occurs:
in the cold-starting of a plant, since all parts of plant are under air pressure before being filled condensate, and it is not possible to remove all the air from the feed water/steam circulation system by means of starting evacuation; PA0 during low-load operation, since, for economic reasons and because of practical problems, the suction capacity of the vacuum pumps for the incoming air cannot be made so great that all the parts of the condense bundle are sufficiently scavenged with steam; PA0 during normal load operation, because, despite a sufficient suction capacity, the concentration of air becomes so great towards the end of condensation (that is to say, towards the air cooler) that measurable oxygen concentrations are obtained; PA0 because even in normal load operation very many condenser designs have zones in which accumulations of air occur; and PA0 because, during the preparation of the make-up water, the purified demineralized make-up water is flushed with air to expel carbon dioxide, and it is therefore saturated 100% with air. PA0 (a) Starting deaerating and low-load deaerating by spraying the recirculated feed water via the condenser tubes. This idea is successful only when the available suction capacity becomes greater than the suction capacity required for oxygen contents of .ltoreq.10 ppb. This can be expected only in the case of loads beyond 30 to 40%, since the spraying of heated-up condensate via the condenser bundles has the desired effect only when the feed water does not pass through any zones of greater air concentrations over the entire path of this condensate. It is also impossible to purify the entire feed water circulation system by recirculating the condensate. PA0 (b) Increasing the suction capacity in the low-load range by reducing the steam content in the suction stream by means of co-condensation. However, co-condensation can only condense some of the steam from the suction mixture, and the suction device still always has to suck up all the air. This is generally possible only at condenser pressures which are above the "no-load pressure" of the condenser. PA0 (c) Subsequent deaerating of the condensate in the hot well by means of packing inserts or by blowing in steam below the water level. This idea will be successful only when sufficient steam is available for being blown in and for heating up the condensate to saturation temperature. This necessitates an external steam source during the cold-starting of the plant. Also, when packed units are used, sufficient height must be available for the packing inserts; when steam is blown in, there must be sufficient covering of the condensate and sufficiently fine steam distribution in the condensate. This solution likewise fails to provide for purification of the feed water circulation system. PA0 (a) purified and deaerated feed water is provided over the entire operating range; PA0 (b) thermal deaerating is possible without the aid of separate facilities, such as, for example, auxiliary steam, so that before steam generation there is largely no need for the provision of, for example, gland-sealing steam or preservation steam; and PA0 (c) the time for deaerating before the plant is started up can be reduced to a minimum.
Copper is dissolved from the wetted metal surfaces of cooper alloys, for example in the presence of ammonia and oxygen, or else is introduced into the feed water as the result of erosion or corrosion of these metal surfaces. Corrosion products and further impurities accumulate in the feed water mainly during shutdown periods.
Experts take the view, at the present time, that it is necessary to aim for a maximum oxygen content of .ltoreq.10 ppb (parts per billion) over the entire operating range of the plant.
In a load range between 40 and 100%, oxygen contents of .ltoreq.5 ppb in the condensate have been detected in good condensers, that is to say those with a proven good deaerating capability. In this case, the make-up water is deaerated in the condenser itself. However, approximately 70 ppb have been measured in the hot well of the condenser during the cold-starting of such a plant and in low-load operation. These oxygen contents should be reduced further.
To solve the corrosion problem, the following proposals were discussed at the EPRI condenser seminar in June 1983 in Orlando, Fla.: